Sophisticated control systems typically use multiple processors. Given the particular control system, these processors may need to exchange information. For example, vehicle control systems typically incorporate at least two processors: an engine control unit and an automatic transmission control unit. Information exchange between the two control units allows finer and more intelligent control of both the engine and transmission. For example, information exchange between the engine control unit and automatic transmission control unit allows the reduction of automatic transmission shift shock by suppressing engine torque.
FIG. 1 illustrates in block diagram form prior circuit 20 for exchanging information between two processors in a master-slave configuration. Processor 22 and processor 24 exchange information via a dual port RAM (DPRAM) 26. Additional circuitry, or "glue logic," converts the multiplexed address and data signals from processors 22 and 24 into demultiplexed address and data signals for DPRAM 26.
A disadvantage of prior circuit 20 is that at any one time, the same piece of data must be stored in three separate memory locations: in both, processors 22 and 24 and within DPRAM 26. This is because when microcontroller 22 wishes to make data available to microcontroller 24, it must first place the information in DPRAM 26. Microcontroller 24 then reads the data from DPRAM 26 and stores it within its own internal RAM.
Yet another disadvantage of the prior circuit is that memory updates are very slow because three memory devices must be used in order to share data between two processors.
Yet another disadvantage of prior circuit 20 for exchanging information between processors is that DPRAMs are very expensive. A single DPRAM may cost as much as twenty dollars.
A still further disadvantage of prior circuit 20 for exchanging information between processors is that it increases the number of address and data buses, which in turn increase the external noise that enters the prior circuit. The noise in turn degrades the performance and reliability of the prior circuit.